Packet-switched telephony is an alternative to traditional circuit-switched telephony. In circuit-switched telephony, a circuit is dedicated to each active telephone call. In packet-switched telephony, the audio signal is partitioned into data “packets” that are individually routed over a data network, sharing the network's pathways with packets from many other sources. Before a packetized audio segment can be played out, its packets must be received and re-assembled.
Packet telephony has several advantages when compared with circuit-switched telephony. Packet telephony uses network resources efficiently, since packets need not be sent when no audio signal is present on the line. Packet telephony also often employs audio codecs (coder/decoder pairs) in order to compress the amount of data that must pass over the network. And with at least some packet telephony systems, users may avoid long distance charges that would be incurred with a circuit-switched connection.
Packet telephony also has disadvantages. Whereas a circuit-switched connection—once established—usually provides fairly consistent connection quality over the length of the call, a packet telephony connection may deliver widely-varying connection quality. Quality variations generally mirror the instantaneous packet delivery performance of the packet-switched connection, which in turn depends on the level of competing traffic. As traffic increases, packets may be delayed, increasing the latency of the connection. An increase in the interarrival jitter, which is an expression of the variation in packet transit times, also increases latency. In some cases, the network or receiver may throw away packets that have been delayed too long, causing dropouts in the reconstructed speech.
A typical packet-switched telephone connection is illustrated in FIG. 1. A calling party 20 connects over a local circuit-switched network 24 to a packet voice gateway 28. After connection, calling party 20 signals the number of a desired party to gateway 28. Gateway 28 attempts to locate a second gateway 30 that is local to the desired party. If such a gateway is found, a packet-switched connection is established with gateway 30 and the desired party's number is communicated to gateway 30. Gateway 30 connects to the desired party 22 through a second local circuit-switched network 26, and the connection is complete.
A typical gateway 28 is illustrated in FIG. 2. The gateway communicates with a circuit-switched network through a front end 54, which typically terminates one or more TDM (time-division multiplexed) digital PCM (pulse-code modulated) circuit-switched telephony signals, such as those following the well-known T1 or E1 standards. On the other end of gateway 28, a packet data transceiver 58 communicates with a packet-switched network, e.g., using IP (Internet Protocol) or ATM (Asynchronous Transfer Mode) packets. In between front end 54 and transceiver 58, one or more signal processors 56 perform audio processing functions such as voice activity detection, DTMF detection, and audio coding and decoding on the audio data streams. A host processor 52 controls and coordinates the operation of gateway 28.